The invention relates to a layered arrangement in solid state systems. The invention also relates to a cryogenic component.
The use of high temperature superconductors for incorporation in cryo-electronic components required the development of integrated multilayer circuits. Examples of superconductor integrated multilayer circuits are SNS-Josephson contacts or flux transformers. In addition, for producing a flux transformer for example, throughgoing contacts or strip conductor crossovers are frequently used.
Josephson contacts, through contacts and conductive strip crossovers are, in addition, the main components for a series of further multilayer components and are significant for cryo-electronics in general. The characteristics of such components in layered technology, for example, Josephson contacts, are a function of the barriers between the superconductive contacts through the barrier material and are not determined by the interfacial resistance of the boundary layer regions between the mutually-contacting layers.
For the desired suppression of interfacial resistances in such contacts, clean ex-situ fabricated boundary layers of high temperature superconductor materials like YBa2Cu3O7-x or PrBa2Cu3O7-x are prepared, for example, by chemical etching in nonaqueous Br-ethanol solutions {M. I. Faley, U. Poppe, H. Soltner, C. L. Jia, M. Siegel, and K. Urban, xe2x80x9cJosephson Junctions, Interconnects and Crossovers on Chemically Etched Edges of YBa2Cu3O7xe2x80x9dxe2x80x9d.xe2x80x94Appl. Phys. Lett., v. 63, No. 15, 2138-2140 (1993)}.
The chemical etching produces not only structurally undamaged boundary surfaces but also small edge angles of about 3xc2x0. The use of these shallow edges after structuring of the high temperature superconductor thin film is significant for two reasons. On the one hand, this edge structure supports current transport in the c-direction with strongly anisotropic high temperature conductors and provides a solution to the problems of current transport. On the other hand, such edge structures allow a relatively homogeneous covering of the base electrode by an insulating or nonsuperconductive layer.
The homogeneous covering or coating of the base electrode by a thin barrier layer is also of significance in the production of Josephson contacts. Ideally through-contacts (vias) should have the smallest possible interfacial resistance while, by comparison, good insulating characteristics are required for them in the production of conductive strip crossovers. Up to now, for insulation, cubic materials like for example SrTiO3 or CeO2 were often used. These materials have the drawback that the superconductive base electrode is frequently deficient in oxygen and may not have sufficient superconductive characteristics. Because of this drawback, the advantageous nonaqueous chemical etching in Br-ethanol for conductive strip crossovers or Josephson contacts cannot be used.
It is therefore the object of the invention to provide a layered arrangement in which the quality of the boundary surface regions between the layers can be adjustable in a targeted manner in dependence upon the desired boundary conditions.
This object is achieved with a layered arrangement containing at least one layer on the basis of a high temperature superconductive material with at least one CuO2 plane forming unit cells. The layer is bonded to a nonsuperconducting layer. A modified interfacial layer is provided between the two layers. The object is further achieved with a cryogenic component having such a multilayer system.
According to the invention the high temperature superconductive material layer is bonded with a nonsuperconducting layer via a modified interfacial region at least on one of the mutually contacting layers. The surface region or the interfacial layer can be doped with ions, especially with metal ions for the modification. The surface region or the interfacial layer can be implanted with ions, especially with metal ions for the modification. The nonsuperconductive layer on its surface turned away the superconductive layer can also be modified in its surface region. Advantageously the nonsuperconductive layer on its surface turned away the superconductive layer is bonded to a further high temperature superconductive layer. PrBa2Cu3O7-x is a preferred material of the nonsuperconductive layer. A multilayer system with a plurality of layered arrangements as described above can form a through contact arrangement or a conductive strip crossover.
It has been found that in this way it is possible to produce an epitaxial multilayer system with high temperature superconductors for integrated cryo-electronic circuits which is especially suitable for producing Josephson contacts, through contacts or conductive strip crossovers. By contrast with earlier known comparable structures, it is possible to obtain in an advantageous manner improved superconductive characteristics within the overall heterostructure.
The layered arrangement according to the invention or the component according to the invention has a homogeneous epitaxial growth and a complete oxidation of the layer sequence, especially the multilayer structure. These requirements are fulfilled by the use of oxidic materials which are technically, chemically and structurally compatible with one another. It has been found that the oxidic high temperature superconductors basically have a charge carrier density which is comparable with that of highly doped semiconductors. The spatial variation in the charge carrier density, for example at boundary layers, can be, by comparison to customary metals, extend relatively large distances up to about 100 nm into the layer. As a result, the transport characteristics of thin oxidic materials can be strongly influenced by the behavior in the region of the interface between the respective layers.
With the aid of a modification of one or more of these boundary layers and thus of the adjustability of the value of the interfacial resistance over a wide range of values, it is possible to fabricate integrated multilayer systems based upon high temperature superconductors which fulfill the individual function-determining requirements of such cryo-electronic components.
A targeted variation or adjustability of the interfacial resistance can be produced by a targeted manipulation of the boundary layer in this manner. Thus different materials which are technologically, chemically and structurally compatible with one another can be used.